Electroluminescent device

ABSTRACT

The efficiency of gallium phosphide electroluminescent devices, emitting light in the red region of the spectrum, produced by the liquid phase epitaxial deposition of p-type material on an n-type substrate depends in part on the concentration of zinc and oxygen in the gallium solvent used in the deposition and on the heat treatment after deposition. It has been found that inclusion in the gallium of 0.03 mole percent zinc and 0.35 mole percent Ga2O3 lead to the production of mounted devices of greater than 6 percent photon efficiency when junction formation is followed by a suitable heat treating schedule.

United States Patent Saul [4 1 Nov. 21, 1972 [541 ELECTROLUMINESCENTDEVICE 3,462,320 8/1969 Lynch ..l48/ 171 2 inventor: Robert H. Saul, 1Clinton Lane, 3,647,579 2/1972 Ladany ..l48/i7l Scotch Plains NJ 07073,619,304 11/1971 Narto ..l48/171 [22] Filed: March 1972 PrimaryExaminer-Martin H. Edlow 2 App] 233, 30 Attorney-R. .l. Guenther RelatedU.S. Application Data 57] ABSTRACT Division 0? 843,546, g- 3, 1969- Theefficiency of gallium phosphide electroluminescent devices, emittinglight in the red region of [52] 317/235 N1 317/235 the spectrum,produced by the liquid phase epitaxial 317/235 AQ deposition of p-typematerial on an n-type substrate [51] Int. Cl. ..H01l 15/00 depends inpan on the concentration f zinc and [58] Field of Search "317/235 235235 AN ygen in the gallium solvent used in the deposition and on theheat treatment after deposition. It has been References C'ted found thatinclusion in the gallium of 0 03 mole per- UNITED STATES PATENTS centzinc and 0.35 mole percent (321,0 lead to the production of mounteddevices of greater than 6 per- 3,555,283 1/1971 Gnmmelss "250/217 centphoton efficiency when junction formation is fol- 3,365,630 1/1968 Logan..317/237 lowed by a suitable heat treating Schedule 3,592,704 7/1971Logan ..l48/l71 3,470,038 9/1969 Logan 148/171 2 Claims, 4 DrawingFigures N-TYPE SG SUBSTRATE**N-TYPE LPE LAYER (T9 P-TYPE LPE LAYER I as0.2 Z I l 4 i i l I i l T x( 70 "60 '50 -40 -?0 '20 IO I0 5O l I +o.2 32I ---0.4 1 l I 0.6 l I I l I 33 l l PATENTEDnnv21 Ian 3.703.671

SHEET 1 UF 2 Z :3 2 0.4 LL u. 0.2

0'0 llllnll] ||||l1l1| 0.0l OJ LO MOLE PERCENT Ga 0 |N SQLUTION 2 HEATTREATED EFFlCIENCY(/o) NOT HEAT TREATED O 0.0l 0.! L0 MOLE PERCENTZn INSOLUTION ELECTROLUMINESCENT DEVICE This application is a division ofapplication Ser. No. 848,546, filed Aug. 8, 1969.

BACKGROUND OF THE INVENTION 1. Field of the Invention This disclosurepertains to the production of gallium phosphide electroluminescent lightsources.

2. Description of the Prior Art Electroluminescent p-n junction deviceswhich emit under forward bias conditions are under active developmentfor a variety of usages as indicator lights and as elements in morecomplex visual displays. In such devices light is generated during theprocess of electron-hole recombination.

Materials Gallium phosphide (GaP) has proven useful aselectroluminescent material in the visible region of the spectrum. Itbelongs to the class of indirect band gap semiconductors which meansthat the electron-hole recombination requires the presence of a thirdbody such as a dislocation, a vacancy, a substitution or interstitialimpurity or some deviation from a perfectly ordered crystal. In GaPdevices the third body needed for recombination with the emission of redlight is believed to be an impurity complex consisting of an oxygen ionand an acceptor ion (most commonly Zn or Cd) which are presentsubstitutionally in the crystal lattice as a nearest neighbor pair onthe p-side of the p-n junction.

Under the influence of an electric field in the forward bias directionan electron is injected from the nregion into the p-region where it istrapped by the complex. Subsequently. a hole is trapped at the samesite, recombining with the electron and emitting a photon of red light.If there are no complexes present in the region of injection, theelectron will, in time, recombine by one of a number of other processeswhich do not involve the emission of visible light. Thus, an efficient(GaP electroluminescent device requires both the efficient injection ofelectrons into the p-region and the presence, in the region ofinjection, of a sufficient concentration of oxygen-acceptor complexes.

GaP is a Ill-V compound semiconductor whose constituents belong tocolumn three and five of the periodic table of the elements. The donor(n-typ dopants are usually selected from column six and are included inthe crystal lattice in a minus 2 ionic state and the acceptor dopantsare usually selected from column two and are included in the crystallattice in the plus 2 ionic state. However, the amphoteric dopants fromcolumn four are sometimes used, their valence state being determined bythe particular substitutional site occupied. The most widely used donordopants are sulphur (S), selenium (Se) and telurium (Te) while the mostwidely used acceptor dopants are zinc (Zn) and cadmium (Cd). Theamphoteric dopants Si and Sn have recently attracted some interest.

Growth Techniques A number of different techniques have been employed inthe fabrication of Ga? electroluminescent devices. The techniques mostpertinent to this disclosure involve the epitaxial deposition ofmaterial of one conductivity type from a liquid Ga solution upon asubstrate of the other conductivity type. A p-n junction, so produced isknown as epitaxially grown junction. Substrates have been produced bytechniques such as the Csochrollski technique (crystal pulling from a(is? melt), solution growth (the slow cooling of a solution of GaP andsuitable dopants in molten gallium), vapor phase epitaxy (the epitaxialdeposition of Ga? and suitable dopants from a carrier gas onto a GaAssubstrate, which is subsequently ground off) and liquid phase epitaxy(LPE) (to be described below.

In an exemplary form of the LPE process as applied to GaP (Lorenz andPilkuhm, .lour Appl Phys, 37 (i966) 4094) a suitable substrate is heldat the upper end of a tube. In the lower end are placed carefullymeasured quantities of gallium (as the solvent), the required Ga? andthe desired dopants (as solutes). The temperature of the tube is raisedto between 1,000C and l,200C where the constituents dissolve in the mo]-ten gallium. The tube is then rotated or tipped so that the molten massflows over the substrate and the temperature is lowered at a controlledrate. As the temperature of the molten mass decreases, the dissolvedmaterial goes out of solution and is deposited on the substrate as anepitaxial crystal. This process has been referred to as "tipping."

Doping Levels Such electroluminescent devices form an active field ofresearch, much of this research going into an effort to optimize theconcentrations of the various dopants on the p and n sides of the p-njunction. In an attempt to simplify the experimental conditions andoptimize the Zn and 0 concentrations in the p-type material without thepresence of the n-type layer, photoluminescent measurements wereperformed in which electrons were "injected" into the conduction handthrough excitation by high energy light (Gershenzon et al., Jour of ApplPhys, 37 (l966) 483). These experiments showed, for solution grownmaterial, the optimum concentration of Zn in the gallium solution to bein the range 0.1 mole percent to II mole percent relative to the galliumsolvent (see above reference FIG. 2), and the optimum concentration ofGa,0, (as the source of 0 doping) to be in the range of 0.003 molepercent to 0.1 mole percent (see above reference page 1,533). Later workby other investigators was strongly influenced by these findings takingthese as the optimum concentration ranges. Some of this subsequent workinvolved the LPE process (Lorenz and Pilkuhn, .lour Appl Phys, 37 (l966)4094; Logan et al., Appl Phys Lett, 10 (I967) 206; Shih et al. Jour ApplPhys, 39 (1968) 2747; Allen et al., Jour Appl Phys, 39 (1968)2977;Ladany, Jour Electrochem. Soc, 116 (1969) 993).

The optimum donor conentration on the n side of the junction isinfluenced by the following two factors. As large an electron density aspossible is desired for efficient electron injection from the n-side tothe p-side. However, if the electron concentration is too high then-type material becomes absorptive of the generated light. Thisabsorption is important since a large proportion of the generated lightis internally reflected at the surface of the device and traverses thedevice several times before emerging. It has been found that the optimumdonor concentration lies in the range of 0.3 X 10" to L0 X 10 per cubiccentimeter in the n-type material (Kressel et al. Solid State Elect, ll(i968) 467). This work was done using tellurium as a donor.

However, sulphur and selenium have been shown to be essentiallyequivalent as donor dopants. Heat Treatment The heat treatment ofdevices of this class after junction formation has been shown to bebeneficial. The amount of benefit derived. however, has variedconsiderably. Logan et al. (Appl Phys Lett, 10 (1967) 206), whoinvestigated devices made by LPE of Te doped n-type material on Zn anddoped p-type substrates, heat treated their devices at temperaturesbetween 450C and 725 C for times greater than 16 hours. They reportincreases of as much as an order of magnitude in the efficiency of theirdevices. Their maximum efficiencies were between 1 percent and 2percent. Shih et al. (Jour Appl Phys, 39 (1968) 2747) and Allen et al.(Jour Appl Phys, 39 (1968) 2977) investigating devices made by the LPEof Zn and 0 doped p-type material on the doped n-type substratesrealized improvements of, at most, a factor of two reaching efficienciesof at most 1 percent.

SUMMARY OF THE INVENTION The inventive matter disclosed here pertains toa process for the process for the production of diodes with efficienciesin the 4 percent to 7 percent range. This breakthrough couldsignificantly influence the solid state visual display industry. It hasbeen found that such efficiencies can be realized by departing from theheretofore accepted optimum concentration ranges in the direction oflower Zn and higher 0 (in the form of Ga,0, in the LPE solution). Thesedevices are made by the LPE of a Zn and 0 doped p-type GaP layer on antype substrate where the gallium solvent contains Zn in theconcentration range 0.02 mole percent to 0.06 mole percent relative tothe gallium and Ga O in the concentration range 0.25 mole percent to 1mole percent for an LPE process starting at l,060C. The aboveefficiencies are realized when the donor concentration in the n-typesubstrate falls within the prior art optimum range and the resultingdevice is heat treated at temperatures within the range of 450C to 800Cfor times between 3 hours and 60 hours.

The above exemplary processes have produced devices containing 1 X 10"to X per cubic centimeter of O donors and 3 X l0" to l X l0" per cubiccentimeter of Zn acceptors within the first 10 microns of the p-side ofthe p-n junction and 0.3 X 10" to 2 X l0 per cubic centimeter of Tewithin the first 10 microns of the n-side of the p-n junction. Theseregions are the critical regions for the light production and it isclear that the teaching of this disclosure extends, beyond the currentprocesses used to realize these preferred concentrations, to any processby which these concentrations can be produced.

In addition to the aforementioned dopants, it may be necessary to addother donor or acceptor dopants to modify bulk semiconducting propertiesof the device such as resistivity. Another class of possible inclusionsare isoelectronic materials such as GaAs which act as neither acceptorsnor donors but act to change the semiconducting band gaps and mayinfluence such properties as the wavelength of the emitted light. GaP-GaAs mixed crystals (until the composition 40% GaP 60% GaAs) areindirect band gap semiconductors maintaining a GaP like character. It isintended to include devices with such additional dopants within theteaching of this disclosure. Definition Efficiency when used in thisdisclosure efficiency is to be taken to means the ratio between thenumber of photons of light emitted from the device and the total numberof charge carriers (electrons plus holes) passing through the devicesacross the light emitting pa junction. This is sometimes referred to asthe "external quantum efficiency" of the device and is greater than thetrue energy efficiency by approximately the ratio between the band gapenergy and energy of the photon. For devices such as those disclosedhere the quantum efficiency is of the order of 20 percent higher thanthe true energy efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a curve showing theefficiency (vertical axis) of Ga! electroluminescent devices formed bythe LPE deposition of a p-type layer on an n-type substrate, as afunction of the amount of Ga,0, in the solution (horizontal axis). Theamount of Zn is held fixed at 0.16 mole percent of the solvent;

FIG. 2 is a set of two curves showing the efficiency (vertical axis) ofGa? electroluminescent devices formed as above, as a function of theamount of Zn in the solution (horizontal axis) for heat treated andunheat treated devices. The amount of Ga,0, is held fixed at 0.35 molepercent of the solvent;

FIG. 3 is a curve showing the concentration of the various dopants in arepresentative high efficiency device as a function of position in thedevice, forming a concentration profile. Donor concentrations are shownabove the horizontal axis and acceptor concentrations are shown belowthe horizontal axis; and

FIG. 4 is a perspective view partly in section of a capsule used for theLPE deposition process.

DETAILED DESCRIPTION OF THE INVENTION The Inventive Process Duringexperiments into the production of GaP electroluminescent devices by theLPE of a p-type layer on an n-type substrate it was decided to venturebeyond the limits of G310; doping which had theretofore been consideredoptimum. Indeed the upper end of this range had been shown to correspondto the maximum equilibrium solubility at the LPE growth temperature(Foster et al. .lour Electrochem Soc 116 (1969) 494). Using a 0.16 molepercent Zn doping (within the prior art range) it was found that, for anLPE process starting at 1,060C, device efficiency increasedmonotonically with Ga,O, doping (see FIG. 1) until the neighborhood of 1mole percent beyond which sufficiently perfect epitaxial layers were notobtainable in the apparatus used. The upper limit, thus, does notrepresent an optimum but merely a practical limit imposed by theapparatus. Choosing a concentration of 0.35 mole percent Ga,0, (wellwithin the newly found desirable range) further experiment showedremarkable results. Devices made using different Zn dopings showed abroad efficiency maximum below 0.1 mole percent Zn (see FIG. 2, curve 1)before heat treatment. The heat treatment of devices using the prior artZn doping yielded modest efficiency improvement. However, the efficiencyimprovement afforded by heat treatment increased dramatically as the Zndoping was decreased reaching a peak of a factor greater than 4 at 0.03mole percent (see FIG. 2, curve 2.)

The measurements indicated in FIGS. 1 and 2 were made on devices in atest jig with simple pressure contacts. After heat treatment the peakefficiency was greater than any previously reported GaP device. Whenthese devices were provided with ohmic contacts by the usual gold alloybonding techniques and encapsulated, as is common practice, in a dome oftransparent high index of refraction (1.6) material, the max imumobserved efficiency rose to 7.2 percent. Alloy bonding reduces resistivelosses and the high index dome reduces the effects of total internalreflection. The efficiencies of representative encapsulated devices overthe Zn doping range are indicated in parenthesis in FIG. 2.

The LPE process described above as a preferred embodiment of theinvention started from a temperature of l,060C. This process, however,can be initiated over a wide range of temperature limited, at the lowend, by the solubility of the various solutes and, at the high end, bythe vapor pressure of phosphorus (35 atmospheres at the melting point ofGaP 1 ,700). The temperature interval l000 C to l200 C represents aworkable range over which experiments have been performed. Operation atthe temperatures higher than l,060C should lead to the solution of moreof the Sa o, and should permit reliable crystal growth to the order of 2mole percent Ga o The distribution coefficient at Zn at these highertemperatures favors inclusion of more Zn in the solid extending thepreferred Zn concentration range down to 0.0] mole percent depending onthe initial temperature.

Heat Treatment Logan et al. in their investigations of LPE n on pdevices found that these devices benefited from heat treatment in thetemperature range 400C to 725C for 16 hours or longer. The experimentsreferred to in this disclosure for LPE p-n devices agree in major partwith these findings, but an attempt was made to establish morerestricted preferred ranges. The knowledge of such restricted ranges isbeneficial since the use of excessively high temperatures leads to anincreased danger of contamination and the use of excessively lowtemperatures requires inordinately long treatment times. It was found tobe unnecessary to heat treat at temperatures above 600C and it was foundthat heat treatment at 500C required longer than l8 hours (overnight),but less than 60 hours (over a weekend) to achieve best results.

A preferred schedule was developed which minimized both time andtemperature of the treatment. This schedule consists of treatment at 600C for five hours followed by a treatment at 500C for 18 hours. Theseparticular times were chosen to fit conveniently within a 24 hours day.It is clear that they do not represent an optimum but merely indicatethe desirability of a heat treatment with an initial period above 550Cand a terminal period below 550C (the temperature need not be constantduring these periods). These results probably indicate the presence ofat least two types of diffusion processes (e.g. annealing of de- 6 fectsand formation of Zn-() nearest neighbor complexes) during the heattreatment one having the higher threshold energy than the other.

Concentration Profiles The concentrations of the various dopants in thefinished devices has been determined from a series of capacitancemeasurements made on angle-lapped devices. In order to perform such ameasurement, the device in the region of the junction is lapped at asmall angle to the plane of the p-n junction. An array of gold dots isthen deposited on the lapped face forming an array ofmetal-semiconductor diodes. The net dopant concentration as a functionof position in the device, the concentration profile, can be derivedfrom a-c and d-c capacitance measurements of the diodes (LA. CopelandTRANS lEEE, ED-l6 I969), 445).

[f a region of the device contains only one active dopant (i.e., donoror acceptor) then the above measurement will give the concentration ofthat dopant directly. If a region contains more than one active dopant aseries of measurements on different devices will be necessary. Forinstance, if the n-type region is doped with only Te (a donor)capacitance measurements will give the Te concentration profiledirectly. However, if the p-type region of the operative device is dopedwith ZN and 0 measurements on two devices will be needed.

first and inoperative device is formed as is the operative device butwith the ornmission of the Ga,0; doping. From this the Zn (an acceptor)concentration profile is derived (by the above capacitance measurementson an angle lapped device). The operative device is then examined. Since0 is a donor, compensation will take place and the net acceptorconcentration in the p-type region of the operative device will be lessthan the Zn concentration in the p-type region of the inoperativedevice. The difference between these concentrations is the O donorconcentration. The above measurement technique is best known at thepresent time but, clearly not the only possible technique.

FIG. 3 shows the concentration profile of a typical high efficiencydevice. This device was formed by the LPE deposition of a p-type layerof Zn and 0 doped GaP onto a composite substrate formed by the LPEdeposition of an n-type layer of Te doped GaP on an ntype solution grownsubstrate lightly doped with Te. The Te concentration in the n-type LPElayer 34 increases to 0.9 X l0 per cubic centimeter at the junctionwhile the net acceptor concentration 32 is 0.42 X [0" per cubiccentimeter starts at 0.4 X l0" per cubic centimeter. Measurement of adevice made with no 0 doping showed a Zn concentration 33 starting at0.58 X l0" per cubic centimeter implying that the operative device hasan O donor concentration of L6 X 10'' per cubic centimeter. Since thelengths characteristic of the electron and hole transport processes areof the order of l to 4 microns in Gal, the material within l0 microns ofthe n-side of the p-n junction 31 supplies most of the injectedelectrons and most of the light is produced within 10 microns of thep-side of the p-n junction 31. Thus, the doping concentration of primaryimportance are those within 10 microns of each side of the junction 31.FIG. 3 shows that an exemplary high efiiciency device has, in region ofthe p-n junction, a Te concentration in the n-type material of 0.9 X 10'per cubic centimeter, a Zn concentration in the p-type material of 5.5 X10 X 10 per cubic centimeter and a concentration of 0 in the p-typematerial of 1.5 X l0" per cubic centimeter.

The doping concentrations away from the junction effect the deviceefficiency in a secondary way. Since the light produced near thejunction must pass through this material in order to emerge from thedevice (indeed, internal reflection may cause some of the light totraverse the device several times before emerging) efficiency will beadversely effected if the material away from the junction is absorptiveof the light. Free carriers absorb red light so that it is desirable toproduce a device in which the concentration of dopants decreases awayfrom the junction. From this point of view it is believed that anefi'icient device would have, as the composite n type substrate, a thinlayer (perhaps 10 microns) of heavily Te doped GaP (perhaps 2 X 10 percubic centimeters) deposited on a lightly doped substrate and a p-typeregion doped with as much Zn and O as possible, consistant with a closecompensation of the Zn by the O. This would provide, in the region ofthe junction 31, a large concentration of electrons on the n siderelative to holes on the p side for efficient injection and a largeconcentration of Zn-O pairs for efficient light emission. Away from thejunction, the free carrier concentration is low, thus the lightabsorption would be small.

Exemplary Procedure Following is a procedure which is exemplary of thosewhich can be used to produce the electroluminescent device referred toin this disclosure. The procedure can be referred to, briefly, as a p-ndouble tipping done in a sealed fused silica capsule on a solution grownsubstrate and incorporating an in situ heat treatment. The capsule usedis shown in FIG. 4. A fused silica tube 41 is provided with a sealingplug 45 and holds a fused silica boat 43. The capsule 41, held at anangle, and the substrate 42 is placed in the upper end of the boat. Thelower end of the boat 43 contains the mass 44 of the solvent gallium,Gal and the appropriate dopants.

For the first deposition (or tipping) the substrate is a lightly Tedoped solution grown Gal substrate which has been ground and polished onthe phosphorus-( 1 l 1) face. After suitable cleaning procedures, 0.015mole percent Te and 6.5 mole percent Gal are added to 6 grams of Ga toform the LPE solution. Epitaxy then is produced under a forming gasatmosphere starting at l,060C by tipping and cooling, the forming gasbeing necessary to reduce transport of the substrate via gaseous GaTe.After the completion of the deposition, the crystal is recovered bydigesting the Ga in warm nitric acid. The resulting composite substrateis then polished for use in the p-tipping.

For the deposition of the p-type layer a 6 gram Ga charge is doped with6.5 mole percent GaP, 0.03 mole percent Zn and 0.35 mole percent oa,0,.The capsule is evacuated and epitaxy proceeds as above. l-leat treatmentcan take place in situ by arresting the cooling cycle for hours at 600Cand 18 hours (overnight) at 500C. The most efficient devices have beenproduced using this in situ heat treatment but other measurementsindicate that heat treatment after recovery of the crystal is alsoeffective.

After recovery of the crystal by digestion, mesa diodes of approximately7 X 10" cm junction area are fabricated and mounted on a gold platedT018 diode mount using a pressure contact. This is used as a test ilarfifiiiiffiiin ivfr'ff iii Bfiig'fljifiii'llliii wires to the n-typelayers. The bonded diodes are then encapsulated in a dome of high indexof refraction (1.6) transparent epoxy to reduce the effects of totalinternal reflection.

Comments on the Scope of the Invention Much of the above material hasbeen illustrative and included only to add to the clarity of theteaching. Many variations in the materials used, the depositiontechniques and the device fabrication techniques leave the basic dopantconcentration dependence of the device efficiency unaffected. Thesubstrate may be doped with donors other than Te and may be produced onany of the other processes known in the art. The utility of otheracceptor dopants in the deposited layer has been disclosed earlier, butin addition, Ga O, is only one of the several possible sources of Odoping. Among the others is ZnO.

The details of the LPE process are subject to much variation. As analternative to tipping" such processes as the mechanical lowering of thesubstrate into the solution (dipping) are under investigation. Thesealed capsule arrangement has been included in this disclosure as apreferred embodiment since it is considered to lead to a morecontrollable and reproducable process than the open tube arrangement inwhich an inert or reducing gas passes through the deposition capsule. Inthe open tube arrangement, however, consideration must be given to thepossible loss of dopants into the gas stream during the depositioncycle. Such variations of the LPE process do not avoid the utilizationof the teaching of this disclosure.

The devices described in the exemplary experimental procedure were mesadiodes. However, the processing of the finished wafer by processes suchas scribing and cracking leave the basic production of light uneffected.Either before or after the production of the light producing p-njunction disclosed here, other rectifying junctions or other of the manyforms of electrical contact known in the art may be introduced in orderto form a multicontact device whose light-producing junction is stilltaught here.

What is claimed is:

1. An electroluminescent device composed principally of Ga? containingat least one p-n junction characterized in that the material within thefirst 10 microns of the p side of the said p-n junction contains atleast an average concentration of between I X 10" per cubic centimeterand 9 X 10" per cubic centimeter of O donors and a concentration ofbetween 2 X 10" per cubic centimeter and l X 10" per cubic centimeter ofan acceptor selected from the group Zn and Cd.

2. A device of claim 1 in which the material within the first 10 micronsof the n side of the said p-n junction contains at least one of theelements S, Se, Si, Sn and Te as the major dopant in an averageconcentration within the range 0.3 X l0 per cubic centimeter to 2 X 10"per cubic centimeter.

# i i i

1. An electroluminescent device composed principally of GaP containingat least one p-n junction characterized in that the material within thefirst 10 microns of the p side of the said p-n junction contains atleast an average concentration of between 1 X 1017 per cubic centimeterand 9 X 1017 per cubic centimeter of O donors and a concentration ofbetween 2 X 1017 per cubic centimeter and 1 X 1018 per cubic centimeterof an acceptor selected from the group Zn and Cd.